The present invention relates to a horizontal type light-emitting diode (referred to as an LED hereinafter) to be used as connected to an electric wiring board in a state in which its pn junction plane is perpendicular to the wiring surface of the electric wiring board, a method for manufacturing the LED and a method for connecting the LED to the electric wiring board.
A great many LEDs are used for self-light-emitting type display devices that can be seen clearly even in a dark place or as back illuminators of display devices. In particular, the LEDs are considered most promising as small-size thin type display devices and the back illuminators thereof for use in narrow spaces of letter display sections or number buttons of portable telephone units, finders of cameras and the like.
The LED chip itself has a small side dimension of not greater than 300 .mu.m. In order to make good use of the properties of the LED, the prior art reference of Japanese Patent Laid-Open Publication No. SHO 57-49284 discloses a horizontal type LED that needs no connection of a metal wire and connects its pn junction plane perpendicular to the wiring surface of an electric wiring board.
FIG. 9 is a perspective view showing a state in which a conventional LED is mounted horizontally on an electric wiring board. In FIG. 9 are shown an n-type semiconductor layer 401, a p-type semiconductor layer 402, a negative electrode 403 and a positive electrode 404. The layers 401 through 404 constitute the conventional LED 400. When connecting the LED 400 to an electric wiring board 408, there have been the processes of temporarily fixing the LED 400 onto the electric wiring board 408 with an adhesive 410 that has an electrical insulating property, connecting the negative electrode 403 to a wiring section 409a of the electric wiring board 408 by means of solder 405 that serves as a brazing material and connecting the positive electrode 404 to a wiring section 409b of the electric wiring board 408 by means of solder 406 that serves as a brazing material. The adhesive 410 is an insulator, and if a pn junction plane 420 is covered with the adhesive 410, then there is the effect of preventing the LED 400 from being damaged through short circuit in the soldering stage.
However, the LED 400 is formed by laminating the n-type semiconductor layer 401 and the p-type semiconductor layer 402 on a semiconductor wafer (not shown) that has a circular shape of a diameter of about 50 mm (or a rectangular shape of a side dimension of about 10 mm) and dicing the semiconductor wafer into squares of a size of about 300 .mu.m.
Therefore, if the LED 400 is used in a horizontal posture, light is taken out from a side surface 407 of the LED 400, the side surface 407 being brought in contact with a dicing blade. The n-type semiconductor layer 401 and the p-type semiconductor layer 402 of the LED 400 have high refractive indices. Therefore, light generated from the pn junction plane 420 does not go outward since it is reflected inside except for light roughly perpendicularly incident on the side surfaces 407a, 407b and 407c that are the surfaces from which light is taken out.
The above phenomenon will be described with reference to FIG. 10. As shown in FIG. 10, if light A generated in a portion that belongs to the pn junction plane 420 and is located apart from the side surface 407 is emitted in a direction inclined at an angle of not smaller than .PHI. relative to the pn junction plane 420, then the light is incident on the positive electrode 404 and disappears through dispersion, absorption and the like. For example, assuming that the pn junction plane 420 has a size D of 300 .mu.m and the p-type semiconductor layer 402 being transparent to the emitted light has a thickness t of 2 .mu.m, then the light that can go out of the side surface 407 without being incident on the positive electrode 404 is only the light that is emitted at an angle of not greater than 0.4.degree. relative to the pn junction plane 420.
Conversely, the light that is not incident on the positive electrode 404 even when inclined at an angle .theta. of 10.degree. relative to the pn junction plane 420 is the light generated at a point that belongs to the pn junction plane 420 and located within a distance d of not greater than about 10 .mu.m from the side surface 407. The light emitted at the inclination angle of 10.degree. relative to the pn junction plane 420 is to be inclined at an angle of about 35.degree. relative to the junction plane when radiated outward as refracted at the side surface 407, the angle of about 35.degree. being approximately equal to the angle of radiation of the LED 400. In other words, it can be appreciated that the light can be efficiently taken out when it is generated at a depth d of not greater than about 10 .mu.m from the side surface 407.
On the other hand, the light emission efficiency of the pn junction plane 420 in the vicinity of the side surface 407 of the LED 400 is lowered due to mechanical damage in the dicing stage. For this reason, if the conventional LED is used in the horizontal posture, then the light emission efficiency is apparently reduced. Therefore, it is proper to etch the portion in the vicinity of the mechanically damaged side surface 407 by immersing the semiconductor in an acid liquid that dissolves the semiconductor. However, there has been the problem that the negative electrode 403 and the positive electrode 404 are also dissolved for the reasons described later.
If the LED is used in the horizontal posture, both the positive electrode and the negative electrode are connected to the wiring sections of the electric wiring board by means of silver paste or solder. Particularly in the case of an LED to be used in a place that may have an elevated temperature, the solder should preferably be used for the connection.
When performing connection by means of solder, the positive electrode 404 and the negative electrode 403 should preferably be provided by a metal such as gold (Au) that has a good affinity with solder and is not eroded by acid. However, it is also well known that, if the positive electrode 404 and the negative electrode 403 are made of only Au, the metal is totally dissolved in the solder, resulting in failed soldering. For this reason, it is a usual practice to employ nickel (Ni) that has a good affinity with solder and cover its surface with Au for the prevention of the oxidation of Ni. However, Ni is eroded by acid. Accordingly, there has been the problem that Ni exposed on the side surfaces is dissolved by the acid when the wafer is divided into small pieces of individual LEDs by dicing and the side surfaces are etched by being immersed in an acid liquid and then falls off together with Au located on Ni.
As shown in FIG. 9, in order to prevent the n-type semiconductor layer 401 and the p-type semiconductor layer 402 of the LED 400 from being damaged through short circuit when connecting the wiring sections 409a and 409b with the positive electrode 404 and the negative electrode 403 of the LED 400 with solders 406 and 405, respectively, the pn junction plane 420 is covered with the adhesive 410 that is an insulator. However, if the quantity of adhesive 410 is small, the portion that belongs to the pn junction plane 420 and is exposed on the side surface 407 cannot be completely covered, frequently causing damage and short circuit of the LED 400. The pn junction plane 420 of the LED 400 is normally located several micrometers to several tens of micrometers apart from the surface of the negative electrode 403 or the positive electrode 404. Therefore, if the quantity of adhesive 410 is increased so as to ensure the covering, then the adhesive 410 adheres to the wiring section 409a or the wiring section 409b, resulting in the failed adhesion of the brazing material. This also leads to the problem that the pn junction plane is hard to be protected by controlling the quantity of the adhesive 410.